Combined input and output queue for packet forwarding in network devices

ABSTRACT

An apparatus for switching network traffic includes an ingress packet forwarding engine and an egress packet forwarding engine. The ingress packet forwarding engine is configured to determine, in response to receiving a network packet, an egress packet forwarding engine for outputting the network packet and enqueue the network packet in a virtual output queue. The egress packet forwarding engine is configured to output, in response to a first scheduling event and to the ingress packet forwarding engine, information indicating the network packet in the virtual output queue and that the network packet is to be enqueued at an output queue for an output port of the egress packet forwarding engine. The ingress packet forwarding engine is further configured to dequeue, in response to receiving the information, the network packet from the virtual output queue and enqueue the network packet to the output queue.

This application is a continuation of U.S. application Ser. No.16/709,611, filed Dec. 10, 2019, the entire content of which is hereinincorporated by reference.

TECHNICAL FIELD

This disclosure generally relates to computing devices and,particularly, queuing in networking devices.

BACKGROUND

A computer network is a collection of interconnected network devicesthat can exchange data and share resources. Example network devicesinclude routers, switches, and other layer two (L2) network devices thatoperate within layer two of the Open Systems Interconnection (OSI)reference model, i.e., the data link layer, and layer three (L3) networkdevices that operate within layer three of the OSI reference model,i.e., the network layer. Network devices within computer networks ofteninclude a control unit that provides control plane functionality for thenetwork device and forwarding components for routing or switching dataunits.

A router of a computer network may use one or more queues facilitatebandwidth matching between senders and receivers. The network switch mayreceive packets from a sender and transmit a portion of the receivedpackets from the sender to a receiver, storing a remaining portion ofthe received packets in the queue. After transmitting the portion of thereceived packets, the network switch transmits the remaining portions ofthe packets stored in the queue to the receiver.

In general, it is often desirable to partition and allocate theresources so as to achieve different Quality of Service (QoS) fordifferent types of packet flows. For example, in response to receiving apacket from the source device, a network device may classify the packetas originating from the source device and identifies a particularpriority provisioned in the router for the source device. The networkdevice then typically stores the network packet as one or more discretedata units in one of a number of queues that is associated with thedetermined priority. The router services the queues in a manner thatsatisfies each of the defined priorities. For example, during operation,the network device performs dequeue operations in accordance with theprovisioned priorities to select network packets from the queues toschedule delivery of the network packets. In some instances, the networkdevice executes multiple dequeue operations in an overlapping manner toconcurrently select and service multiple queues. As one example, anetwork device may implement dequeue and network packet schedulingoperations in a pipelined fashion such that multiple network packets arebeing dequeued and scheduled for processing at any given point in time.By executing these dequeue operations concurrently, the network deviceimproves the number of network packets that can be serviced in a givenamount of time.

SUMMARY

In general, this disclosure describes techniques for improved queueingsystems in network devices. A network device, such as a router or aswitch, may enqueue network packets in one or more queues prior toswitching internally between packet forwarding engines, or prior totransmitting the packets over the network. A queueing system for anetwork device may be configured to combine elements of a virtual outputqueue (VOQ) and combined input output queue (CIOQ). As used herein, aVOQ may refer to a buffer at an ingress side, where each input portmaintains a separate virtual queue for each output port. However,maintaining a separate virtual queue for each output port does not scalewell. In contrast, a CIOQ may be configured to buffer at an egress side.However, a CIOQ may limit throughput of the network device due tohead-of-line (HOL) blocking. For instance, CIOQ may limit throughput ofthe network device due to HOL blocking across a switching fabric when aninput queue aggregates multiple flows intending to reach an egresspacket forwarding engine or when multiple ingress packet forwardingengines try to reach the same egress packet forwarding engine and thebandwidth of the egress packet forwarding engine is exceeded.

In accordance with the techniques of the disclosure, a network devicemay be configured to provide “CIOQ behavior” that enables output queuescaling. For example, the network device may be configured to usevirtual output queuing at egress as the local output queue. Forinstance, the network device may be configured to enqueue a networkpacket at a VOQ for a packet forwarding engine and the packet forwardingengine will schedule the network packet to be enqueued at a particularport of the packet forwarding engine. In this instance, network devicemay “loopback” information indicating the network packet in the virtualoutput queue and that the network packet is to be enqueued at as outputqueue for the particular port. In this way, the network device may allowqueue scale to increase as more packet forwarding engines are added tothe system while helping to minimizing head-of-line blocking across theswitch fabric.

In one example, an apparatus for switching network traffic includes aningress packet forwarding engine implemented in circuitry and configuredto: determine, in response to receiving a network packet, an egresspacket forwarding engine for outputting the network packet; and enqueuethe network packet in a virtual output queue for output to the egresspacket forwarding engine; the egress packet forwarding engineimplemented in processing circuitry and configured to, in response to afirst scheduling event, output, to the ingress packet forwarding engine,information indicating the network packet in the virtual output queueand that the network packet is to be enqueued at an output queue for anoutput port of the egress packet forwarding engine; wherein the ingresspacket forwarding engine is further configured to, in response toreceiving the information: dequeue the network packet from the virtualoutput queue; and enqueue the network packet to the output queue; andwherein the egress packet forwarding engine is further configured to, inresponse to a second scheduling event that is after the first schedulingevent: dequeue the network packet from the output queue; and output thenetwork packet at the output port.

In another example, a method includes: determining, in response toreceiving a network packet and by an ingress packet forwarding engineimplemented in processing circuitry, an egress packet forwarding enginefor outputting the network packet; enqueuing, by the ingress packetforwarding engine, the network packet in a virtual output queue foroutput to the egress packet forwarding engine; outputting, in responseto a first scheduling event and by the egress packet forwarding engineimplemented in processing circuitry, to the ingress packet forwardingengine, information indicating the network packet in the virtual outputqueue and that the network packet is to be enqueued at an output queuefor an output port of the egress packet forwarding engine; dequeuing, inresponse to receiving the information and by the ingress packetforwarding engine, the network packet from the virtual output queue andenqueuing, by the ingress packet forwarding engine, the network packetto the output queue; and dequeuing, in response to a second schedulingevent that is after the first scheduling event and by the egress packetforwarding engine, the network packet from the output queue andoutputting, by the egress packet forwarding engine, the network packetat the output port.

In another example, an apparatus for switching network traffic includes:a plurality of interface cards; an ingress packet forwarding engineimplemented in circuitry and configured to: determine, in response toreceiving a network packet with the plurality of interface cards, anegress packet forwarding engine for outputting the network packet; andenqueue the network packet in a virtual output queue for output to theegress packet forwarding engine; the egress packet forwarding engineimplemented in processing circuitry and configured to, in response to afirst scheduling event, output, to the ingress packet forwarding engine,information indicating the network packet in the virtual output queueand that the network packet is to be enqueued at an output queue for anoutput port of the egress packet forwarding engine; wherein the ingresspacket forwarding engine is further configured to, in response toreceiving the information: dequeue the network packet from the virtualoutput queue; and enqueue the network packet to the output queue; andwherein the egress packet forwarding engine is further configured to, inresponse to a second scheduling event that is after the first schedulingevent: dequeue the network packet from the output queue; and output,with the plurality of interface cards, the network packet at the outputport.

In one example, an apparatus includes: means for determining, inresponse to receiving a network packet, an egress packet forwardingengine for outputting the network packet; means for enqueuing thenetwork packet in a virtual output queue for output to the egress packetforwarding engine; means for outputting, in response to a firstscheduling event, information indicating the network packet in thevirtual output queue and that the network packet is to be enqueued at anoutput queue for an output port of the egress packet forwarding engine;means for dequeuing, in response to receiving the information, thenetwork packet from the virtual output queue and means for enqueuing thenetwork packet to the output queue; and means for dequeuing, in responseto a second scheduling event that is after the first scheduling event,the network packet from the output queue and means for outputting thenetwork packet at the output port.

The details of one or more examples of the techniques of this disclosureare set forth in the accompanying drawings and the description below.Other features, objects, and advantages of the techniques will beapparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example network in accordancewith the techniques of the disclosure.

FIG. 2 is a block diagram illustrating an example router within thenetwork of FIG. 1 in accordance with the techniques of the disclosure.

FIG. 3 is a block diagram illustrating an example shaper credit updateengine within the router of FIG. 2 in accordance with the techniques ofthe disclosure.

FIG. 4 is a block diagram illustrating an example router switching localnetwork traffic using network packet loopback in accordance withtechniques of this disclosure.

FIG. 5 is a block diagram illustrating an example router switchingfabric network traffic to egress using network packet loopback inaccordance with techniques of this disclosure.

FIG. 6 is a block diagram illustrating an example router switchingfabric network traffic from ingress using network packet loopback inaccordance with techniques of this disclosure.

FIG. 7 is a block diagram illustrating an example router switching localnetwork traffic using metadata loopback in accordance with techniques ofthis disclosure.

FIG. 8 is a block diagram illustrating an example router switchingfabric network traffic from ingress using metadata loopback inaccordance with techniques of this disclosure.

FIG. 9 is a block diagram illustrating an example process for switchingnetwork traffic in accordance with techniques of this disclosure.

Like reference characters refer to like elements throughout the figuresand description.

DETAILED DESCRIPTION

Some systems use a Combined Input and Output Queuing (CIOQ) for a largequeue scale. In CIOQ systems, a packet buffer, such as, for example, aDelay Bandwidth Buffer (DBB) and Output Queues (OQ) are held at egressalong with Congestion Management (CM) and hierarchical scheduling. Insome examples, a CIOQ system may include fine grain queueing at ingress.Accordingly, in CIOQ, small fabric Input Queues (IQ) may be used atingress (e.g., one per destination packet forwarding engine). With CIOQ,the queue scale increases as more packet forwarding engines are added tothe system, because network packets fanout at egress per queue.

However, CIOQ systems may suffer from fabric congestion that may beaddressed, for example, using fabric overspeed (e.g., 2X) to satisfyQuality of Service (QoS) targets. Fabric congestion may occur whenmultiple input ports on different packet forwarding engines attempt toreach the same egress OQ. In this example, the CIOQ system may dropnetwork packets at the ingress fabric interface. The ingress fabricqueues may aggregate the traffic through relatively small queues (e.g.,stored in On-Chip-Memory (OCM)) with only a few priority constrains andwithout per-queue QoS guarantees.

To avoid the foregoing difficulties associated with CIOQ systems, somenetwork devices may use virtual output queuing. In virtual outputqueuing, each ingress packet forwarding engine includes a virtual outputqueue (VOQ) that uniquely identifies an Egress OQ. A VOQ (e.g., one VOQon each packet forwarding engine) may combine with an OQ to form thequeue. The VOQ on ingress may provide a delay bandwidth buffer with onlya small OQ at egress (e.g., only at a head of the queue is available ategress for scheduling to a port). Because the DBB is kept at ingress invirtual output queuing, such systems may omit techniques for mitigatinghead-of-line blocking across the switch fabric. Such head-of-lineblocking may be due to the egress scheduling of the VOQ, which togetherwith fair fabric scheduling may use little or no overspeed.

Virtual output queuing, however, may lack OQ scaling. Because eachingress packet forwarding engine may use a VOQ for each egress OQ, thenumber of VOQ on the ingress packet forwarding engine determines amaximum OQ scale. As such, rather than increasing queue scale as morepacket forwarding engines are added, in the case of CIOQ systems, theVOQ system may have a maximum number of OQ that is determined by thenumber of VOQ on the ingress packet forwarding engine (e.g., 48,000OQs), considering that ingress memory committed to queueing is finiteand may not be easily upgraded to add additional storage capacity.Because the total number of OQ does not grow with the addition of packetforwarding engines, the average number of OQ per packet forwardingengine becomes smaller as more packet forwarding engines are added to asystem. For a hierarchical quality of service solution, systems usingvirtual output queues may limit a number of packet forwarding enginessupported in a system to a small number (e.g., 4 or 8 packet forwardingengines).

Techniques described herein describe a system configured to provide“CIOQ behavior” that enables OQ scaling in stand-alone (e.g., combinedbuffer (CBUF) local switching) and fabric based systems. In someexamples, a system may use virtual output queues at egress as the OQ.For instance, a system may enqueue a network packet at a VOQ for apacket forwarding engine and the packet forwarding engine will schedulethe network packet to be enqueued at a particular port of the packetforwarding engine. In this instance, the system may “loopback”information (e.g., packet header with packet payload, only meta data,etc.) indicating the network packet in the virtual output queue and thatthe network packet is to be enqueued as an output queue for theparticular port. Techniques described may include a system that mayloopback the information locally using metadata. In some examples, asystem may loopback the information by looping back the network packetwith a header and a packet payload for the network packet. In this way,the system may allow queue scale to increase as more packet forwardingengines are added to the system while helping to minimize head-of-lineblocking across the switch fabric. For example, the system may have somehead-of-line blocking if multiple flows aggregate through a singleingress queue on an ingress packet forwarding engine but are laterseparated out into separate output queues at an egress packet forwardingengine.

FIG. 1 is a block diagram illustrating an exemplary system 102 in whichnetwork 104 includes routers 106A-106B (collectively, routers 106).Devices 110A-110N (collectively, devices 110) connect to network 104 viarouters 106 in order to access resources provided by network 104. Eachof devices 110 may be an end-user computing device, such as a personalcomputer, a laptop computer, a mobile telephone, a network telephone, atelevision set-top box, a video game system, a point-of-sale device, apersonal digital assistant, an intermediate network device, a networkappliance, a supercomputer, a mainframe computer, an industrial robot,or another type of device capable of interfacing with and communicatingover network 104.

Network 104 may include a plurality of network devices that facilitatethe access of content by devices 110. Each of the plurality of networkdevices may comprise one of a router (e.g., routers 106), a switch, aserver, a database server, a hub, a firewall, an IntrusionDetection/Prevention (IDP) device and/or any other type of networkingequipment or device that facilitates the transfer of data to and fromdevices 110. Network 104 includes routers 106, which communicate usingvarious protocols, such as the Border Gateway Protocol and the InternetControl Message Protocol, in order to exchange routing, networkconfiguration information, and other information. The network may be alocal area network (“LAN”), such as a token ring or Ethernet network, avirtual local area network (“VLAN”), or another type of network. Thenetwork may comprise one or more wired or wireless links. For example,network 104 may be an Ethernet network that comprises one or moreEthernet cables. In another example, the network may be a WirelessFidelity (“Wi-Fi”) network that uses wireless radio transmissions tocommunicate information. In another example, network 104 may be a mobilenetwork. Although shown as a single network 104 in FIG. 1, network 104may comprise any number of interconnected networks, either public orprivate, in which the various networks interconnect to form one or morevirtual networks.

Network 104 provides a variety of resources that may be accessed bydevices 110. In the example of FIG. 1, network 104 includes contentserver 112 that stores or otherwise sources content, which, as the termis used herein, refers to any data commonly transmitted and/or storedwithin a network, such as web-based applications, images, documents, webpages, video data, audio data such as voice, web-based games, scripts,or any other type of network-based content. Network 104 may supportmulticast techniques to improve the delivery efficiency of datatransmitted with the network. Typically network 104 will also connect toa variety of other types of devices (e.g., file servers, printers,telephones, and e-mail and other application servers). Network 104 isalso shown coupled to public network 114 (e.g., the Internet) via router106B. Public network 114 may include, for example, one or more clientcomputing devices. Public network 114 may provide access to web servers,application servers, public databases, media servers, end-user devices,and many other types of network resource devices and content.

Network 104 may transmit content to devices 110 through router 106Ausing one or more packet-based protocols, such as an Internet Protocol(IP)/Transmission Control Protocol (TCP). In this respect, network 104may support the transmission of data via discrete data units, oftenreferred to as “network packets,” or simply “packets.” As a result,network 104 may be referred to as a “packet-based” or “packet switched”network. While described in this disclosure as transmitting, conveying,or otherwise supporting packets, network 104 may transmit data accordingto any other discrete data unit defined by any other protocol, such as acell defined by the Asynchronous Transfer Mode (ATM) protocol, or adatagram defined by the User Datagram Protocol (UDP).

Network traffic delivered by network 104 may be classified according toa number of categories. For instance, content server 112 may stream livevideo to one of devices 110 through router 106A. Packets that transmitsuch video may be classified as streaming multimedia packets. Contentserver 112 may also send web pages to one of devices 110 using HTTPpackets. As another example, information exchanged by routers 106 may becategorized as network management traffic. In addition to beingclassified by application, network traffic may be classified by sourceor destination, user, protocol, and port (for TCP and UDP), among otherscharacteristics.

Various categories of network traffic may require a certain level ofnetwork performance. For example, streaming multimedia may requireguaranteed bandwidth to provide an acceptable user experience. Asanother example, network management traffic should experience low delaysin order to maintain the efficiency of a network. Also, internet serviceproviders (ISPs) may prioritize traffic for certain users over othersbased on a service provider agreement. To meet these requirements,network 104 includes mechanisms to support Quality of Service (QoS)guarantees according to a number of predefined QoS levels.

Routers 106 receive, analyze, and classify packets to assign the packetsto a suitable priority level. In addition to classifying packets,routers 106 process the received and classified packets according totheir priority level. In this manner, routers 106 implement aspects ofthe QoS guarantees provided by network 104. In addition, based oninformation received from other devices in system 102, routers 106determine the appropriate route through the system for each receivedpacket and forwards the packet accordingly.

Routers 106 may regulate a speed at which packets are transmitted toprevent flooding on the network. For example, routers 106 may include atoken bucket shaper that spends “tokens” to dequeue a correspondingamount of bytes from a queue and transmit them over the network, and maynot transmit packets if the token bucket shaper has insufficient tokensto spend. In other words, each token may correspond to a number of bytesthat the token bucket shaper is permitted to dequeue from the queue andtransmit over the network. In this way, the token bucket shaper acts toregulate the speed at which packets are removed from the queue andtransmitted on the network.

Some routers may use a CIOQ techniques, that use a delay bandwidthbuffer and output queues that hold packets at egress along withcongestion management and hierarchical scheduling. In this example, aCIOQ system may use fabric input queues at ingress (e.g., one perdestination packet forwarding engine). As such, the queue scaleincreases as more packet forwarding engines are added to the CIOQsystem, because network packets fanout at egress per queue. However,CIOQ systems may suffer from fabric congestion that may be addressed,for example, using fabric overspeed (e.g., 2X) to satisfy Quality ofService (QoS) targets. Fabric congestion may occur when multiple inputports on different packet forwarding engines attempt to reach the sameegress OQ. In this example, the CIOQ system may drop network packets atthe ingress fabric interface. The ingress fabric queues may aggregatethe traffic through relatively small queues (e.g., stored inOn-Chip-Memory (OCM)) with only a few priority constrains and withoutper-queue QoS guarantees.

Some network devices may use virtual output queuing. In virtual outputqueuing, a router may use a virtual output queue (VOQ) that uniquelyidentifies an egress OQ. A VOQ (e.g., one VOQ on each packet forwardingengine) may combine with an OQ to form the queue. The VOQ on ingress mayprovide a delay bandwidth buffer with only a small OQ at egress (e.g.,only at a head of the queue is available at egress for scheduling to aport). Because the delay bandwidth buffer is kept at ingress in virtualoutput queuing, systems using VOQ techniques may omit techniques formitigating head-of-line blocking across the switch fabric. Routersconfigured to use virtual output queuing, however, may lack OQ scaling.Because each ingress packet forwarding engine may use a VOQ for eachegress OQ, the number of VOQ on the ingress packet forwarding enginedetermines a maximum OQ scale.

In accordance with the techniques of the disclosure, routers 106 may beconfigured to provide “CIOQ behavior” that enables OQ scaling instand-alone (e.g., combined buffer (CBUF) local switching) and fabricbased systems. For example, router 106A may be configured to use virtualoutput queues at egress as the OQ. For instance, router 106A may beconfigured to enqueue a network packet at a VOQ for an egress packetforwarding engine and the egress packet forwarding engine will schedulethe network packet to be enqueued at a particular port of the packetforwarding engine. In this instance, router 106A may “loopback,” to aningress packet forwarding engine, information indicating the networkpacket in the virtual output queue and that the network packet is to beenqueued at an output queue for the particular port. Techniquesdescribed may include a system that may loopback the information locallyusing metadata. In some examples, router 106A may loopback theinformation by looping back the network packet with a header and apacket payload for the network packet. In this way, router 106A mayallow queue scale to increase as more packet forwarding engines areadded to the system while helping to minimize head-of-line blockingacross the switch fabric.

In operation, router 106A may determine, in response to receiving anetwork packet, an egress packet forwarding engine for outputting thenetwork packet. For example, router 106A may determine an egress packetforwarding engine of router 106A. In some examples, router 106A maydetermine the egress packet forwarding engine that corresponds to a nexthop. For instance, router 106A may determine, in response to determiningthat a packet label of the network packet specifies an IP address, anext-hop for the network packet. In this instance, router 106A maydetermine the egress packet forwarding engine assigned to a port thatcorresponds to the next-hop for the network packet.

Router 106A may enqueue the network packet in a virtual output queue foroutput to the egress packet forwarding engine. For example, router 106Astores the network packet (e.g., packet payload, packet header, etc.) atthe virtual output queue. In response to a first scheduling event,router 106A may output, to the ingress packet forwarding engine,information indicating the network packet in the virtual output queueand that the network packet is to be enqueued at an output queue for anoutput port of the egress packet forwarding engine. For example, router106A may determine, using quality of service for different types ofpacket flows and/or a dequeue rate, to schedule the network packet forqueueing by the egress packet forwarding engine for processing by theegress packet forwarding engine. To output the information, router 106Amay output a network packet via a port of the egress router to a port ofthe ingress router a packet payload for the network packet and a headerfor the network packet that includes the information. In some examples,to output the information, router 106A may output metadata using localswitching (e.g., using a combined buffer) without outputting the packetpayload.

In response to receiving the information, the ingress packet forwardingengine of router 106A may dequeue the network packet from the virtualoutput queue. For example, router 106A may remove the network packet(e.g., packet payload and packet header) and/or a pointer representingthe network packet from the virtual output queue. Router 106A mayenqueue the network packet to the output queue. For example, router 106Amay add the network packet and/or a pointer representing the networkpacket to the output queue.

In response to a second scheduling event that is after the firstscheduling event, router 106A may dequeue the network packet from theoutput queue and output the network packet at the output port. Forexample, router 106A may determine, using quality of service fordifferent types of packet flows and/or a dequeue rate, to schedule thenetwork packet for queueing by the egress packet forwarding engine at anoutput queue for the output port. In response to the second schedulingevent, router 106A may output the network packet (e.g., packet payloadand packet header) at the output port and remove the network packet(e.g., packet payload and packet header) and/or a pointer representingthe network packet from the output queue for the output port.

In this way, router 106A may have higher scalability compared to routersthat use VOQ. For example, using techniques described herein, router106A may increase the output queue scale, which may help to support alarger number of customers, thereby improving an operation of a router.For example, assuming each packet forwarding engine of four packetforwarding engines supports 48,000 queues, the combination of the fourpacket forwarding engines using VOQ techniques may support only 48,000queues. However, in accordance with the techniques of the disclosure,the combination of four packet forwarding engines using combined buffertechniques may support 192,000 queues (i.e., 4×48,000), which may allowthe router to support additional customers and, therefore, may improve aperformance of router 106A. In some examples, router 106A may have alower product cost compared to routers configured to use CIOQ.Additionally, techniques described herein may be used with VOQtechniques and/or CIOQ. For instance, router 106A may use VOQ forinternet facing traffic and techniques described herein using a combinedbuffer for inbound traffic from the internet (e.g., for customerqueueing).

Routers 106 may use techniques described herein to use a virtual outputqueue as an output queue. However, in some examples, some of routers 106may use other techniques, such as, for example, virtual output queueing,CIOQ, or another queueing technique. Although the principles describedherein are discussed with reference to routers 106, other networkdevices, such as, for example, but not limited to, an AsynchronousTransfer Mode (ATM) switch, a local area network (LAN) switch, aninterface card, a gateway, a firewall, or another device of system 102may determine a predicted lifetime.

FIG. 2 is a block diagram illustrating an example router 206 withinnetwork 104 of FIG. 1 in accordance with the techniques of thedisclosure. In general, router 206 may operate substantially similar torouters 106 of FIG. 1. In this example, router 206 includes interfacecards 230A-230N (“IFCs 230”) that receive network packets via incominglinks 232A-232N (“incoming links 232”) and send network packets viaoutbound links 234A-234N (“outbound links 234”). IFCs 230 may be coupledto links 232, 234 via a number of interface ports. Router 206 mayinclude a control unit 222 that determines routes of received packetsand forwards the packets accordingly via IFCs 230, in communication withcontrol unit 222.

Control unit 222 includes a routing engine 224 and a packet forwardingengine 226. Routing engine 224 operates as the control plane for router206 and includes an operating system (not shown) that provides amulti-tasking operating environment for execution of a number ofconcurrent processes. Routing engine 224, for example, executes softwareinstructions to implement one or more control plane networking protocols246. For example, protocols 246 may include one or more routingprotocols, such as BGP 252, for exchanging routing information withother routing devices and for updating routing information base (RIB)242. Protocols 246 may further include transport protocols, such asMultiprotocol Label Switching (MPLS) protocol 250, and multicastmanagement protocols, such as Internet Group Management Protocol (IGMP)256. In other examples, protocols 246 may include other routing,transport, management, or communication protocols.

In some examples, routing engine 224 includes command line interface(CLI) 240 to permit an administrator to configure and/or manage router206. For example, the administrator may, via CLI 240, access queuemanager 264 to configure one or more parameters of packet forwardingengines 226. In another example, routing engine 224 includes a graphicaluser interface (GUI) instead of a CLI. In a still further example,routing engine executes Simple Network Management Protocol (SMNP) 254 topermit the administrator to configure and/or control router 206 from aremote terminal.

Routing protocol daemon (RPD) 244 may execute BGP 252 or other routingprotocols to update RIB 242. RIB 242 describes a topology of thecomputer network in which router 206 resides, and also includes routesthrough the computer network. RIB 242 describes various routes withinthe computer network, and the appropriate next hops for each route,i.e., the neighboring routing devices along each of the routes. RPD 244analyzes information stored in RIB 242 and generates forwardinginformation for packet forwarding engine 226, which stores theforwarding information in forwarding information base (FIB) 260. RPD 244may, in other words, resolve routing information stored by RIB 242 toobtain the forwarding information identifying a next hop for eachdestination within the network, storing the forwarding information toFIB 260.

Combined buffer (“CBUF”) 265 may act as queue storage for packetforwarding engines 226 of router 206. CBUF 265 may include local memory(e.g., on chip memory (OCM)) and/or external memory (e.g., HighBandwidth Memory (HBM)). In accordance with the techniques of thedisclosure, CBUF 265 may store queues for router 206. In some examples,CBUF 265 comprises random access memory (RAM), read only memory (ROM),programmable read only memory (PROM), erasable programmable read onlymemory (EPROM), electronically erasable programmable read only memory(EEPROM), flash memory, comprising executable instructions for causingthe one or more processors to perform the actions attributed to them.

CBUF 265 may include one or more queues that are a first-in first-out(FIFO) data structure for organization and temporary storage of data. Inthe example of FIG. 2, queues of CBUF 265 may store one or more networkpackets for router 206. For example, router 206 may store the one ormore packets in one or more queues of CBUF 265 prior to switchinginternally between packet forwarding engines 226. In another example,router 206 may store the one or more packets in one or more queues ofCBUF 265 prior to transmitting the network packets over the network.

For example, CBUF 265 may include virtual output queues 227A-227N(collectively referred to herein as “VOQs 227) and/or output queues219A-219N (collectively referred to herein as “OQs 219). In someexamples, each VOQ of VOQs 227 may be assigned to a respective packetforwarding engine of packet forwarding engines 226. For instance, VOQ227A may be assigned to a first packet forwarding engine of packetforwarding engines 226, VOQ 227B may be assigned to a second packetforwarding engine of packet forwarding engines 226, and so on. Each OQof OQs 219 may be assigned to a respective port of packet forwardingengine of packet forwarding engines 226. For instance, OQ 219A may beassigned to a first port of a first packet forwarding engine of packetforwarding engines 226, OQ 219B may be assigned to a second port of thefirst packet forwarding engine, and so on.

Packet forwarding engine 226 operates as the data plane for router 206and includes FIB 260, shaper credit update engine 262 and queue manager264. Packet forwarding engine 226, for example, processes packetsforwarded and received by router 206 via IFCs 230. For example, packetforwarding engine 226 may apply filters and routing policies to outgoingpackets and forward the packets to a next hop in the network. In someexamples, control unit 222 includes a plurality of packet forwardingengines, each of which are configured to operate similar to packetforwarding engine 226 to provide packet forwarding functions fordifferent flows of network traffic. As used herein, ingress packetforwarding engine and egress packet forwarding engine are merely termsfor providing context relating to a specific network packet. That is,all packet forwarding engines 226 may act as an ingress packetforwarding engine when receiving packets and an egress packet forwardingengine when transmitting network packets. In some examples, a singlepacket forwarding engine may act as both the ingress packet forwardingengine and the egress packet forwarding engine for a single packet.

FIB 260 may associate, for example, network destinations for networktraffic with specific next hops and corresponding IFCs 230 and physicaloutput ports for output links 234. FIB 260 may be a radix treeprogrammed into dedicated forwarding chips, a series of tables, acomplex database, a link list, a radix tree, a database, a flat file, orvarious other data structures. In some examples, FIB 260 includes lookupstructures. Lookup structures may, given a key, such as an address,provide one or more values. In some examples, the one or more values maybe one or more next hops. A next hop may be implemented as microcode,which when executed, performs one or more operations. One or more nexthops may be “chained,” such that a set of chained next hops perform aset of operations for respective different next hops when executed.Examples of such operations may include applying one or more services toa network packet, dropping a network packet, and/or forwarding a networkpacket using an interface and/or interface identified by the one or morenext hops. As shown, FIB 260 may include an Ingress Packet Processor(IPP) 261 and an Egress Packet Processor (EPP 263). IPP 261 maydetermine a packet forwarding engine of packet forwarding engines 226that acts as an egress packet forwarding engine for a network packet.EPP 263 may determine output queue forward statistics at egress.

Queue manager 264 of packet forwarding engine 226 may work with shapercredit update engine 262 to perform management functions for VOQs 227and OQs 219. For example, shaper credit update engine 262 may implementtoken bucket shaper data structures to determine dequeue rates for VOQs227. In this example, queue manager 264 may regulate a flow of networkpackets to VOQs 227 using the dequeue rates specified by shaper creditupdate engine 262. Similarly, shaper credit update engine 262 mayimplement token bucket shaper data structures to determine dequeue ratesfor OQs 219. In this example, queue manager 264 regulates the flow ofnetwork packets from OQs 219 using the dequeue rates specified by shapercredit update engine 262. Shaper credit update engine 262 is describedfurther with reference to FIG. 3.

In accordance with techniques described herein, packet forwardingengines 226 may be configured to reuse VOQs 227 (e.g., ingress VOQs) asegress OQs. For instance, a first packet forwarding engine of packetforwarding engines 226 may act as an ingress packet forwarding enginethat uses VOQ 227A as an ingress VOQ. In this example, VOQ 227 may be“reused” as an egress OQ by a second packet forwarding engine of packetforwarding engines 226. In this instance, the egress packet forwardingengine may “loopback,” to the ingress packet forwarding engine,information indicating the network packet in VOQ 227 and that thenetwork packet is to be enqueued at an OQ 219A for the particular port.In some examples, one or more packet forwarding engines of packetforwarding engines 226 may use VOQs 227 as VOQs at ingress. Because thetotal number of OQ grows with the addition of packet forwarding engines,the average number of OQ per packet forwarding engine becomes larger asmore packet forwarding engines are added to a system. In this way, OQscaling may occur when multiple packet forwarding engines are added to asystem (e.g., 40,000 per packet forwarding engine).

Each OQ of OQs 219 may be scheduled to an egress port, which may placequeueing at an egress port. Ingress packet forwarding engines of packetforwarding engines 226 may support a small number of fabric input queue(e.g., one of VOQs 227) per destination packet forwarding engine withOQ. For instance, queue manager 264 may use one VOQ per destinationpacket forwarding engine loopback channel with priority (e.g., up to 8Priority per destination packet forwarding engine). In some examples,IPP 261 may perform a lookup to determine an egress packet forwardingengine of packet forwarding engines 226 and OQ (e.g., VOQ) per egressoutput port. For instance, IPP 261 may insert a VOQ number for an OQ ategress in a network packet prepend sent from ingress to egress.

Shaper credit update engine 262 may include a Grant Scheduler (GS) 272,which is also referred to herein as “scheduler 272” at egress, which maybe configured to schedule network packets from VOQ to fabric and from OQto the port. For instance, scheduler 272 may schedule fabricpackets/pages from an ingress packet forwarding engine to an egresspacket forwarding engine. Scheduler 272 may be configured to schedulenetwork packets from OQ to the port on egress packet forwarding engine.

Scheduler 272 may directly schedule an egress packet forwarding enginewith OQ to the port. For example, scheduler 272 may include an 8K Queueand there are five 8K Queues (e.g., 5×8K is 40K OQ per packet forwardingengine). In this instance, scheduler 272 may not use fabric forscheduling to port for deterministic behavior.

Router 206 may be configured to support mixing of VOQ and OQ in a samesystem. For example, router 206 may be configured such that some fabricdestinations may be “typical” VOQ/OQ combinations when small queues perport are supported and other destinations may be OQ when larger queuesper port are needed. In some examples, router 206 may use existingpacket loopback paths on an egress packet forwarding engine to enqueuepackets into an OQ with minimal changes to design to support OQ, but atpossible reduced bandwidth (e.g., see FIGS. 4, 5, 6). In some examples,router 206 may be configured such that network packets arrive fromfabric are looped back on egress packet forwarding engine to performdrop check and enqueue (NQ) to the OQ without any new data path needed.In this example, scheduler 272 may schedule OQ direct to port.

Router 206 may use dedicated data paths at egress to achieve fullperformance (e.g., see FIGS. 6, 7, 8). For example, router 206 may beconfigured such that network packets arrive from fabric are firstchecked for admittance to OQ using drop check, and if allowed areenqueued to the OQ. In this example, scheduler 272 may schedule from OQdirect to port. In this way, techniques described herein may use “localswitching” at an egress packet forwarding engine, which may allownetwork packets to be received at ingress and move directly to egresswithout passing through fabric.

In operation, an ingress packet forwarding engine of packet forwardingengines 226 may determine, in response to receiving a network packet, anegress packet forwarding engine of packet forwarding engines 226 foroutputting the network packet. For example, the ingress packetforwarding engine may determine an egress packet forwarding engine ofingress packet forwarding engines 226 for outputting the network packet.FIB 260 may determine the egress packet forwarding engine thatcorresponds to a next hop. For instance, FIB 260 may determine, inresponse to determining that a packet label of the network packetspecifies an IP address, a next-hop for the network packet. In thisinstance, FIB 260 may determine an egress packet forwarding engineassigned to a port that corresponds to the next-hop for the networkpacket.

The ingress packet forwarding engine may enqueue the network packet inVOQ 227A for output to the egress packet forwarding engine. For example,the ingress packet forwarding engine stores the network packet (e.g.,packet payload, packet header, etc.) at VOQ 227A. In response to a firstscheduling event, an egress packet forwarding engine of ingress packetforwarding engine 226 may output, to the ingress packet forwardingengine, information indicating the network packet in VOQ 227A and thatthe network packet is to be enqueued at OQ 219A for an output port ofthe egress packet forwarding engine. For example, scheduler 272 maydetermine, using quality of service for different types of packet flowsand/or a dequeue rate, to schedule the network packet for queueing bythe egress packet forwarding engine for processing by the egress packetforwarding engine. To output the information, egress packet forwardingengine may output a network packet via a port of the egress router to aport of the ingress router a packet payload for the network packet and aheader for the network packet that includes the information. In someexamples, to output the information, egress packet forwarding engine mayoutput metadata using local switching (e.g., using CBUF 265) withoutoutputting the packet payload.

In response to receiving the information, the ingress packet forwardingengine may dequeue the network packet from VOQ 227A. For example,ingress packet forwarding engine may remove the network packet (e.g.,packet payload and packet header) and/or a pointer representing thenetwork packet from VOQ 227A. Ingress packet forwarding engine mayenqueue the network packet to OQ 219A. For example, ingress packetforwarding engine may add the network packet and/or a pointerrepresenting the network packet to OQ 219A.

In response to a second scheduling event that is after the firstscheduling event, egress packet forwarding engine may dequeue thenetwork packet from OQ 219A and output the network packet at the outputport (e.g., link 232A with IFC 230A). For example, scheduler 272 maydetermine, using quality of service for different types of packet flowsand/or a dequeue rate, to schedule the network packet for queueing bythe egress packet forwarding engine at OQ 219A for the output port. Inresponse to the second scheduling event, the egress packet forwardingengine may output the network packet (e.g., packet payload and packetheader) at the output port and remove the network packet (e.g., packetpayload and packet header) and/or a pointer representing the networkpacket from OQ 219A for the output port.

In this way, router 206 may have higher scalability compared to routersthat use VOQ. For example, using techniques described herein, router 206may increase the output queue scale, which may help to support a largernumber of customers, thereby improving an operation of a router. Forexample, assuming each packet forwarding engine of four packetforwarding engines supports 48,000 queues, the combination of the fourpacket forwarding engines using VOQ techniques may support only 48,000queues. However, in accordance with the techniques of the disclosure,the combination of four packet forwarding engines using combined buffertechniques may support 192,000 queues (i.e., 4×48,000), which may allowrouter 206 to support additional customers. In some examples, router 206may have a lower product cost compared to routers configured to useCIOQ. Additionally, techniques described herein may be used with VOQtechniques and/or CIOQ. For instance, router 206 may use VOQ forinternet facing traffic and techniques described herein using a combinedbuffer for inbound traffic from the internet (e.g., for customerqueueing).

FIG. 3 is a block diagram illustrating an example shaper credit updateengine 262 of FIG. 2 in accordance with the techniques of thedisclosure. In one example implementation, shaper credit update engine262 includes rate wheel 370 and scheduler 272. Network devices mayinclude shaper credit update engine 262 to regulate a speed at whichpackets are transmitted to prevent flooding on the network.

Rate wheel 370 provides credit updates to scheduler 272. Scheduler 272may use credits to determine when queue/node data structure 388 ispermitted to transmit one or more bytes enqueued by queue/node datastructure 388. In the example of FIG. 3, rate wheel 370 includes rateinstruction 374 and update rate 376. Rate instruction 374 provides rateupdates for “Guaranteed” (G) and “Maximum” (M) credit fields 378 tocredit adder 382 of scheduler 272. G credits may be used to allocate aguaranteed amount of bandwidth to queue/node data structure 388, unlessthe G rate for the network is oversubscribed. M credits may be used as arate limit to prevent queue/node data structure 388 from exceeding aspecified average transmit rate.

In addition, update rate 376 represents a rate at which credits arebeing updated by rate wheel 370. Update rate 376 provides a normalizeddequeuing rate to queue/node data structure 388. In the example of FIG.3, update rate 376 is the inverse of a rate update period for rate wheel370. In some examples, scheduler 272 applies a low-pass filter to smoothinstantaneous changes in the dequeuing rate.

Scheduler 272 includes credit adder 382, credit updater 392, rateupdater 386, and queue/node data structure 388. Credit adder 382 ofscheduler 272, based on input from clip 380, provides additional creditsto rate updater 392 using MUX 384, which in turn provides suchadditional G/M credits 390 to queue/node data structure 388. Dependingon the value of the current credits and clip 380, rate updater 392 mayadd some, all, or none of the credits to G/M credits 390 of queue/nodedata structure 388. Scheduler 272 uses G/M credits 390 to determine whenqueue/node data structure 388 is permitted to transmit. In someexamples, when G/M credits 390 for queue/node data structure 388 arenon-negative, scheduler 272 may dequeue or transmit packets fromqueue/node data structure 388. Upon dequeuing and transmitting thepackets from queue/node data structure 388, credit updater 386 removes acorresponding number of credits from G/M credits 390 for queue/node datastructure 388. Once G/M credits 390 for queue/node data structure 388are negative, queue/node data structure 388 becomes ineligible fordequeuing or transmitting subsequent packets. Upon accumulating anon-negative value of G/M credits 390, queue/node data structure 388again becomes permitted to dequeue or transmit packets.

FIG. 4 is a block diagram illustrating an example first router 206 forswitching local network traffic using loopback in accordance withtechniques of this disclosure. FIG. 4 illustrates ports 407A-407B,409A-409B, 417A-417B, 419A-419B, which may each represent single ports,a port group (PG), or other ports. Additionally, FIG. 4 illustratescongestion manager 405, which may be configured to perform a networkpacket drop check.

Congestion manager 405 may check each network packet that arrives at aqueue (e.g., VOQ 227A, OQ 219A, etc.) for admittance by first learning anetwork packet size for a respective network packet, a priority for therespective network packet, and a drop precedence for the respectivenetwork packet. For example, congestion manager 405 may check eachnetwork packet by looking at a current queue length to see if thenetwork packet would exceed a drop threshold, e.g., not fit. In thisexample, if congestion manager 405 determines that the network packetwould exceed the drop threshold, congestion manager 405 may drop thenetwork packet (e.g., not written to queue). If congestion manager 405determines that the network packet would not exceed the drop threshold(e.g., the network packet is not dropped), congestion manager 405 mayadmit the network packet to the queue. In addition to tail dropthresholds, congestion manager 405 may compare a network packet withWeighted random early detection (WRED) thresholds, which determine arandom probability of dropping based on priority and drop precedence.

As shown, router 206 includes a fabric input 441 configured to receivenetwork packets from the fabric (e.g., network 104, the Internet, etc.)and a fabric output 443 configured to output network packets to thefabric. To avoid the need for an addition drop check and enqueuebandwidth, FIG. 4 shows a router 206 that is configured for egress OQusing loopback on the packet forwarding engines 226. In the example ofFIG. 4, packet forwarding engine 226A may operate as an ingress packetforwarding engine and packet forwarding engine 226B operates an egresspacket forwarding engine. The loopback path 471 on packet forwardingengines 226 may help to preserve existing data paths. In this example,network packets may arrive on packet forwarding engine 226A input portsand router 206 may use “local switching” (e.g., without using fabricinput 441, fabric output 443, etc.) to move to the network packets toegress without the need for fabric. Because half the PGs may be used forloopback to emulate OQ scaling packet forwarding engine, only half of atotal throughput may be used for network packet forwarding. Networkpackets may make two trips through CBUF 265 and ingress packetforwarding engine 226A and egress packet forwarding engine 226B, whichmay reduce the throughput of packet forwarding engines 226.

FIG. 4 shows a local switching example using CBUF 265. In the example ofFIG. 4, the packet is preserved in the header information when loopingback to ingress from egress through a PG (e.g., port 409B). In thisexample, a network packet that comes into an input port (e.g., port407A) at ingress is switched through CBUF 265 to packet forwardingengine 226B that has the destination output port, looped back to ingresspacket forwarding engine 226A and stored in VOQ 227A. Scheduler 272 ategress schedules reading from VOQ 227A to port 419A, which is done viametadata since the actual packet data remains in CBUF 265 until read outat egress before being sent to EPP 263.

In accordance with the techniques of the disclosure, ingress packetforwarding engine 226A, may determine, in response to receiving anetwork packet, an egress packet forwarding engine for outputting anetwork packet. Ingress packet forwarding engine 226A may enqueue thenetwork packet in virtual output queue 227A for output to the egresspacket forwarding engine. For example, ingress packet forwarding engine226A may enqueue the network packet in virtual output queue 227A ofingress packet forwarding engine 226A for output to egress packetforwarding engine 226B.

Egress packet forwarding engine 226B may output, in response to a firstscheduling event (e.g., determined by scheduler 272) and to ingresspacket forwarding engine 226A, the network packet with a headercomprising information indicating the network packet in VOQ 227A andthat the network packet is to be enqueued at OQ 219A for an output portof the egress packet forwarding engine 226B. For instance, scheduler 272may determine the first scheduling event based on a dequeue rate at VOQ227A.

Scheduler 272 may maintain per queue shaping and priority information.When a queue (e.g., VOQ 227A, OQ 219A, etc.) becomes non-empty,scheduler 272 may install the queue in the scheduler hierarchy (e.g.,“enqueue”) at the configured queue priority. When the rate shapingrequirements are not met, e.g., the queue has not transmitted enoughdata yet, and the queue is at the current serviceable priority,scheduler 272 may select the queue for service by scheduler 272 (e.g.,“dequeue”). Once a queue has met the shaping requirement, e.g., theshaping rate is met, scheduler 272 may remove the queue from serviceuntil a time when the queue receives addition or new shaping credits andcan resume transmission again. Scheduler 272 may determine an amount ofshaping credits a queue receives in a time period to determine a ratefor the queue.

Egress packet forwarding engine 226B may output the network packet withthe header from a first port (e.g., port 417B) of the egress packetforwarding engine 226B to a second port (e.g., 409B) of ingress packetforwarding engine 226A. Ingress packet forwarding engine 226B may, inresponse to receiving the network packet with the header, perform a dropcheck for the network packet.

In response to receiving the network packet with the header, ingresspacket forwarding engine 226A may dequeue the network packet fromvirtual output queue 227A and enqueue the network packet to output queue219A. In response to a second scheduling event that is after the firstscheduling event, egress packet forwarding engine 226B may dequeue thenetwork packet from output queue 227A and output the network packet atthe output port (e.g., output port 419A). For instance, scheduler 272may determine the second scheduling event based on a dequeue rate at OQ219A. While scheduler 272 may determine the first scheduling event usinginformation from queues from multiple packet forwarding engines (e.g.,packet forwarding engine 226A, packet forwarding engine 226B, etc.),schedule 272 may determine the second scheduling event using onlyinformation for queues on packet forwarding engine 226B. As such, thefirst scheduling event may be considered a “coarse” scheduling while thesecond scheduling event may be considered a “fine” scheduling.

FIG. 5 is a block diagram illustrating an example router for switchingfabric network traffic to egress using network packet loopback inaccordance with techniques of this disclosure. FIG. 5 shows networkswitching along path 473 where a network packet arrives at port 407A andis stored in CBUF 265, specifically VOQ 227A, at ingress. Sometime later(e.g., during a scheduling event), packet forwarding engine 226A readsthe network packet out of CBUF 265 and outputs the network packet tofabric output 443 to send the network packet across the fabric to adestination packet forwarding engine.

FIG. 6 is a block diagram illustrating an example router for switchingfabric network traffic from ingress using network packet loopback inaccordance with techniques of this disclosure. FIG. 6 shows a networkswitching along path 475 where a network packet arriving (e.g., read byfabric input 441) from across the fabric by scheduler 272 and arrivingat egress packet forwarding engine 226B. Egress packet forwarding engine226B stores the network packet in OQ 219A in CBUF 265. Scheduler 272reads the network packet out and loops the network packet back toingress packet forwarding engine 226A through the egress PG (e.g., port417B) to the ingress PG (e.g., port 407B).

For example, egress packet forwarding engine 226B may output, inresponse to a first scheduling event (e.g., determined by scheduler 272)and to ingress packet forwarding engine 226A, the network packet with aheader comprising information indicating the network packet in VOQ 227Aand that the network packet is to be enqueued at OQ 219A for an outputport of the egress packet forwarding engine 226B. For instance, egresspacket forwarding engine 226B may output the network packet with theheader from a first port (e.g., port 417B) of the egress packetforwarding engine 226B to a second port (e.g., 409B) of ingress packetforwarding engine 226A. Ingress packet forwarding engine 226B may, inresponse to receiving the network packet with the header, perform a dropcheck for the network packet.

In response to receiving the network packet with the header, ingresspacket forwarding engine 226A may dequeue the network packet fromvirtual output queue 227A and enqueue the network packet to output queue219A. For instance, IPP 261 of ingress packet forwarding engine 226Astores the network packet in the ingress VOQ (e.g., VOQ 227). Scheduler272 may later schedule the network packet to the port by the Egress GS(e.g., port 417A). For example, egress packet forwarding engine 226B maydequeue, in response to a second scheduling event that is after thefirst scheduling event, the network packet from output queue 227A andoutput the network packet at the output port (e.g., output port 419A).

FIG. 7 is a block diagram illustrating an example router 206 forswitching local network traffic using metadata loopback in accordancewith techniques of this disclosure. FIG. 7 illustrates an example router206 that uses a loop-back once the packet is stored in CBUF byrecirculating only the packet header and metadata from egress packetforwarding engine 226B to ingress packet forwarding engine 226A. FIG. 7shows router 206 is configured for egress OQ. Network packets thatarrive on ingress input ports (e.g., ports 407A, 407B, 409A, 409B, etc.)may use “local switching” to move to egress without the need for thefabric (e.g., fabric input 441, fabric output 443, etc.).

In accordance with the techniques of the disclosure, ingress packetforwarding engine 226A may determine, in response to receiving a networkpacket, an egress packet forwarding engine for outputting a networkpacket. Ingress packet forwarding engine 226A may enqueue the networkpacket in virtual output queue 227A for output to the egress packetforwarding engine. For example, ingress packet forwarding engine 226Amay enqueue the network packet in virtual output queue 227A for outputto egress packet forwarding engine 226B.

Egress packet forwarding engine 226B may output, to ingress packetforwarding engine 226A, metadata comprising information indicating thenetwork packet in VOQ 227A and that the network packet is to be enqueuedat OQ 219A for an output port (e.g., port 419A) of egress packetforwarding engine egress packet forwarding engine 226B to ingress packetforwarding engine 226A and refrain from outputting the network packet(e.g., packet payload) to ingress packet forwarding engine 226A. Forinstance, egress packet forwarding engine 226B may output the headerdata and/or metadata to the ingress packet forwarding engine using localswitching (e.g., using CBUF 265).

In response to receiving the metadata, ingress packet forwarding engine226A may dequeue the network packet from virtual output queue 227A andenqueue the network packet to output queue 219A. In response to a secondscheduling event that is after the first scheduling event, egress packetforwarding engine 226B may dequeue the network packet from output queue227A and output the network packet at the output port (e.g., output port419A).

FIG. 8 is a block diagram illustrating an example router for switchingfabric network traffic from ingress using metadata loopback inaccordance with techniques of this disclosure. Fabric input 441 receivesnetwork packets from the fabric and congestion manager 405 performs adrop check. If congestion manager 405 determines that the network packetis allowed, ingress packet forwarding engine 226A enqueues the networkpacket to OQ 219A for scheduling to the port (e.g., port 417A). FIG. 8shows path 479 through router 206 to support the combined input queueand OQ model. However, this may use an additional drop check and enqueuebandwidth in order to store the network packet from the fabric in CBUF265 in addition to those packets coming from locally switched ports oningress packet forwarding engine 226A.

Egress packet forwarding engine 226B may output, in response to a firstscheduling event and to ingress packet forwarding engine 226A, metadatacomprising information indicating the network packet in VOQ 227A andthat the network packet is to be enqueued at OQ 219A for an output port(e.g., port 419A) of egress packet forwarding engine egress packetforwarding engine 226B to ingress packet forwarding engine 226A andrefrain from outputting the network packet (e.g., packet payload) toingress packet forwarding engine 226A. For instance, egress packetforwarding engine 226B may output the header data and/or metadata to theingress packet forwarding engine using local switching (e.g., using CBUF265).

In response to receiving the metadata, ingress packet forwarding engine226A may dequeue the network packet from virtual output queue 227A andenqueue the network packet to output queue 219A. In response to a secondscheduling event that is after the first scheduling event, egress packetforwarding engine 226B may dequeue the network packet from output queue227A and output the network packet at the output port (e.g., output port419A).

FIG. 9 is a block diagram illustrating an example first process forswitching network traffic in accordance with techniques of thisdisclosure. Ingress packet forwarding engine 226A may determine, inresponse to receiving a network packet, an egress packet forwardingengine for outputting a network packet (902). For example, ingresspacket forwarding engine 226A may determine an egress packet forwardingengine (e.g., egress packet forwarding engine 226B).

Ingress packet forwarding engine 226A may enqueue the network packet invirtual output queue 227A for output to the egress packet forwardingengine (904). For example, ingress packet forwarding engine 226A mayenqueue the network packet in virtual output queue 227A for output toegress packet forwarding engine 226B. In some examples, VOQ 227A is acombined buffer (e.g., CBUF 265) for ingress packet forwarding engine226A and the set of egress packet forwarding engines. For instance,ingress packet forwarding engine 226A may enqueue the network packet ina first portion of CBUF 265 that is assigned to virtual output queue227A of ingress packet forwarding engine 226A for output to egresspacket forwarding engine 226B.

Egress packet forwarding engine 226B may output, in response to a firstscheduling event and to ingress packet forwarding engine 226A,information indicating the network packet in the virtual output queueand that the network packet is to be enqueued at an output queue for anoutput port of the egress packet forwarding engine 226B (906). Forexample, egress packet forwarding engine 226B may output, to ingresspacket forwarding engine 226A, the network packet with a headercomprising the information. In some examples, egress packet forwardingengine 226B may output, to ingress packet forwarding engine 226A, thenetwork packet with a header comprising the information. For instance,egress packet forwarding engine 226B may output the network packet withthe header from a first port of the egress packet forwarding engine to asecond port of ingress packet forwarding engine 226A. Ingress packetforwarding engine 226B may, in response to receiving the network packetwith the header, perform a drop check for the network packet.

In some examples, egress packet forwarding engine 226B may outputmetadata comprising the information to ingress packet forwarding engine226A and refrain from outputting the network packet to ingress packetforwarding engine 226A. For instance, egress packet forwarding engine226B may output the metadata to the ingress packet forwarding engineusing local switching.

In some examples, scheduler 272 may select the output port from aplurality of output ports at egress packet forwarding engine 226B. Inthis example, scheduler 272 or another component of egress packetforwarding engine 226B may generate the information to specify that thenetwork packet is to be enqueued at the output queue based on theselection of the output port by scheduler 272. In some examples,scheduler 272 may determine the first scheduling event to regulate aspeed at which data is exchanged from router 206. For instance,scheduler 272 may determine the first scheduling event based on adequeue rate at OQ 219A.

Ingress packet forwarding engine 226A may dequeue, in response toreceiving the information, the network packet from virtual output queue227A (908) and enqueue the network packet to output queue 219A (910).For example, ingress packet forwarding engine 226A may dequeue thenetwork packet from the first portion of CBUF 265 assigned to virtualoutput queue 227A. In some examples, ingress packet forwarding engine226A may enqueue the network packet to a second portion of CBUF 265assigned to output queue 219A.

Egress packet forwarding engine 226B may dequeue, in response to asecond scheduling event that is after the first scheduling event, thenetwork packet from output queue 227A (912) and output the networkpacket at the output port (914). For example, egress packet forwardingengine 226B may dequeue the network packet from the second portion ofCBUF 265. In some examples, scheduler 272 may determine the secondscheduling event to regulate a speed at which data is exchanged fromrouter 206. For example, scheduler 272 may determine the secondscheduling event based on a dequeue rate at OQ 219A.

The techniques described in this disclosure may be implemented, at leastin part, in hardware, software, firmware or any combination thereof. Forexample, various aspects of the described techniques may be implementedwithin one or more processors, including one or more microprocessors,digital signal processors (DSPs), application specific integratedcircuits (ASICs), field programmable gate arrays (FPGAs), or any otherequivalent integrated or discrete logic circuitry, as well as anycombinations of such components. The term “processor” or “processingcircuitry” may generally refer to any of the foregoing logic circuitry,alone or in combination with other logic circuitry, or any otherequivalent circuitry. A control unit comprising hardware may alsoperform one or more of the techniques of this disclosure.

Such hardware, software, and firmware may be implemented within the samedevice or within separate devices to support the various operations andfunctions described in this disclosure. In addition, any of thedescribed units, modules or components may be implemented together orseparately as discrete but interoperable logic devices. Depiction ofdifferent features as modules or units is intended to highlightdifferent functional aspects and does not necessarily imply that suchmodules or units must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modules orunits may be performed by separate hardware or software components, orintegrated within common or separate hardware or software components.

The techniques described in this disclosure may also be embodied orencoded in a computer-readable medium, such as a computer-readablestorage medium, containing instructions. Instructions embedded orencoded in a computer-readable storage medium may cause a programmableprocessor, or other processor, to perform the method, e.g., when theinstructions are executed. Computer readable storage media may includerandom access memory (RAM), read only memory (ROM), programmable readonly memory (PROM), EPROM, EEPROM, flash memory, a hard disk, a CD-ROM,a floppy disk, a cassette, magnetic media, optical media, or othercomputer readable media.

1. An apparatus comprising: an ingress packet forwarding engineimplemented in circuitry and configured to: determine, in response toreceiving a network packet, an egress packet forwarding engine foroutputting the network packet; and enqueue the network packet in a firstportion of a combined buffer; the egress packet forwarding engineimplemented in processing circuitry and configured to, in response to afirst scheduling event, output, to the ingress packet forwarding engine,information indicating the network packet in the first portion of thecombined buffer and that the network packet is to be enqueued at anoutput queue for an output port of the egress packet forwarding engine;wherein the ingress packet forwarding engine is further configured to,in response to receiving the information: dequeue the network packetfrom the first portion of the combined buffer; and enqueue the networkpacket to a second portion of the combined buffer assigned to the outputqueue; and wherein the egress packet forwarding engine is furtherconfigured to, in response to a second scheduling event that is afterthe first scheduling event: dequeue the network packet from the secondportion of the combined buffer; and output the network packet at theoutput port.
 2. The apparatus of claim 1, wherein, to output theinformation, the egress packet forwarding engine is configured tooutput, to the ingress packet forwarding engine, the network packet witha header comprising the information.
 3. The apparatus of claim 2,wherein, in response to receiving the network packet with the header,the ingress packet forwarding engine is configured to perform a dropcheck for the network packet.
 4. The apparatus of claim 2, wherein, tooutput the network packet with the header, the egress packet forwardingengine is configured to output the network packet with the header from afirst port of the egress packet forwarding engine to a second port ofthe ingress packet forwarding engine.
 5. The apparatus of claim 1,wherein, to output the information, the egress packet forwarding engineis configured to output metadata comprising the information to theingress packet forwarding engine and refrain from outputting the networkpacket to the ingress packet forwarding engine.
 6. The apparatus ofclaim 5, wherein, to output the metadata, the egress packet forwardingengine is configured to output the metadata to the ingress packetforwarding engine using local switching.
 7. The apparatus of claim 1,comprising the combined buffer.
 8. A method comprising: determining, inresponse to receiving a network packet and by an ingress packetforwarding engine implemented in processing circuitry, an egress packetforwarding engine for outputting the network packet; enqueuing, by theingress packet forwarding engine, the network packet in in a firstportion of a combined buffer; outputting, in response to a firstscheduling event and by the egress packet forwarding engine implementedin processing circuitry, to the ingress packet forwarding engine,information indicating the network packet in the first portion of thecombined buffer and that the network packet is to be enqueued at anoutput queue for an output port of the egress packet forwarding engine;dequeuing, in response to receiving the information and by the ingresspacket forwarding engine, the network packet from the first portion ofthe combined buffer and enqueuing, by the ingress packet forwardingengine, the network packet to a second portion of the combined bufferassigned to the output queue; and dequeuing, in response to a secondscheduling event that is after the first scheduling event and by theegress packet forwarding engine, the network packet from the secondportion of the combined buffer and outputting, by the egress packetforwarding engine, the network packet at the output port.
 9. The methodof claim 8, wherein outputting the information comprises outputting, tothe ingress packet forwarding engine, the network packet with a headercomprising the information.
 10. The method of claim 9, comprisingperforming, in response to receiving the network packet with the headerand by the ingress packet forwarding engine, a drop check for thenetwork packet.
 11. The method of claim 9, wherein outputting thenetwork packet with the header comprises outputting the network packetwith the header from a first port of the egress packet forwarding engineto a second port of the ingress packet forwarding engine.
 12. The methodof claim 8, wherein outputting the information comprises outputtingmetadata comprising the information to the ingress packet forwardingengine and refraining from outputting the network packet to the ingresspacket forwarding engine.
 13. The method of claim 12, wherein outputtingthe metadata comprises outputting the metadata to the ingress packetforwarding engine using local switching.
 14. An apparatus for switchingnetwork traffic, the apparatus comprising: a plurality of interfacecards; an ingress packet forwarding engine implemented in circuitry andconfigured to: determine, in response to receiving a network packet withthe plurality of interface cards, an egress packet forwarding engine foroutputting the network packet; and enqueue the network packet in a firstportion of a combined buffer; the egress packet forwarding engineimplemented in processing circuitry and configured to, in response to afirst scheduling event, output, to the ingress packet forwarding engine,information indicating the network packet in the first portion of thecombined buffer and that the network packet is to be enqueued at anoutput queue for an output port of the egress packet forwarding engine;wherein the ingress packet forwarding engine is further configured to,in response to receiving the information: dequeue the network packetfrom the first portion of the combined buffer; and enqueue the networkpacket to a second portion of the combined buffer assigned to the outputqueue; and wherein the egress packet forwarding engine is furtherconfigured to, in response to a second scheduling event that is afterthe first scheduling event: dequeue the network packet from the secondportion of the combined buffer; and output, with the plurality ofinterface cards, the network packet at the output port.
 15. Theapparatus of claim 14, wherein, to output the information, the egresspacket forwarding engine is configured to output, to the ingress packetforwarding engine, the network packet with a header comprising theinformation.
 16. The apparatus of claim 15, wherein, in response toreceiving the network packet with the header, the ingress packetforwarding engine is configured to perform a drop check for the networkpacket.
 17. The apparatus of claim 15, wherein, to output the networkpacket with the header, the egress packet forwarding engine isconfigured to output the network packet with the header from a firstport of the egress packet forwarding engine to a second port of theingress packet forwarding engine.
 18. The apparatus of claim 14,wherein, to output the information, the egress packet forwarding engineis configured to output metadata comprising the information to theingress packet forwarding engine and refrain from outputting the networkpacket to the ingress packet forwarding engine.
 19. The apparatus ofclaim 18, wherein, to output the metadata, the egress packet forwardingengine is configured to output the metadata to the ingress packetforwarding engine using local switching.
 20. The apparatus of claim 14,comprising the combined buffer.